1. Field of the Embodiments
The present invention relates to a radiation measuring system and methods. More particularly, the present invention relates to radiation measuring apparatus and methods utilizing a single Geiger Mueller tube (GMT) that provides a count range extending from background dose rates on the order of 20×10−6 rads (R) per hour (20 μR/h) to high dose rates on the order of 1000 R/h.
2. Summary of the Existing Art
One of the most common and well-known radiation measuring apparatus is the Geiger counter, sometimes called a Geiger-Mueller counter. The Geiger counter detects ionizing radiation, including gamma rays and x-rays, and beta particles. At the heart of a Geiger counter is a Geiger Mueller tube (GMT), which typically comprises a glass tube about 2 cm in diameter enclosing a metal cylinder, often of copper, about 10 cm long. (Other dimensions are, of course, also commonly used.) A thin metal wire, e.g. of tungsten, passes along the axis of the metal tube. The cylinder and wire are connected through an end wall of the glass tube to a source of electrical voltage. The tube is filled with a gas, usually a mixture of an inert gas, such as argon or neon, and a halogen, such as chlorine or bromine, at a low pressure, e.g., a few centimeters of mercury. A high voltage, e.g. 550 volts, is set up between the cylinder (which functions as the negative electrode or cathode) and the wire (which functions as the positive electrode or anode). This voltage is just a little less than that needed to create an electrical discharge between the two electrodes.
When a charged particle of sufficient energy enters the GMT, it knocks electrons out of the atoms of the gas. These electrons, being negatively charged, are attracted towards the wire anode, and the atoms from which the electrons originated (which become positively charged ions) are attracted towards the cathode. The high voltage established between the anode and cathode creates a high voltage gradient that accelerates the liberated electrons sufficiently to knock further electrons out of atoms, which in turn are accelerated by the high voltage gradient to knock still further electrons out of other atoms, creating an “avalanche” of electrons. As the avalanche of electrons continues, the positive ions are also accelerated towards the cathode wall. These positive ions strike the cathode wall with sufficient energy to release still additional electrons. All of these electrons descend on the anode wire and are detected as a pulse of electric current. The occurrence of this pulse thus indicates that a charged particle has passed through the tube. The electrical pulses can then be amplified and counted, using appropriate electronic counting circuitry, and/or converted to audible sound, to provide a user of the Geiger counter a quantitative and/or qualitative measure of the number of charged particles encountered by the GMT.
Unfortunately, the rate at which charged particles can be detected by the GMT is limited. This is because during a discharge, i.e., during that time during which the electron avalanche is occurring, the GMT is insensitive to further charged particles arriving at the detector. Thus, some means must be employed to stop the electron avalanche, and to prepare the GMT to detect the next arriving charged particle.
One common technique used to help stop the avalanche is to reduce the voltage potential between the anode and cathode. Some reduction of this voltage occurs naturally as the electrical pulse developed on the anode effectively discharges the charged GMT (which may be considered prior to discharge as a charged capacitor). However, it is also known in the art to deliberately decrease the applied voltage potential for a sufficient time to allow the electron avalanche to sweep out of the GMT, at which time the voltage is again raised to a value just a little less than needed to create an electrical discharge. This process is described in U.S. Pat. Nos. 4,605,859 and 4,631,411. Using this prior art “sweep out” process, conventional GMT counters have a recovery time on the order of 10 μsec. Thus, conventional Geiger counters are able to detect charged particles or radiation at a rate that is limited to no more than about 100,000 pulses per second. This recovery time significantly limits the high dosage rate at which radiation can be detected.
Using the “sweep out” process dosage rate is determined by measuring the period of the time between raising the anode voltage to its operating level and receiving the next Geiger pulse, i.e., time-to-count algorithm (or mode). This is a reciprocal rate measurement technique. The statistical accuracy of such measurement is improved by averaging successive measurements, but can only be as accurate as the starting point for measuring the Geiger period. Unfortunately, this starting point, when the anode voltage is precisely at its proper operating value, is not known with a great deal of certainty. Hence, as the Geiger period becomes shorter and shorter (higher and higher dosage rates), the uncertainty of the starting point of the period contributes a larger and larger error to the measurement. Because the measurement thus becomes increasingly inaccurate for short time intervals, reciprocal rate measurement instruments of the early prior art employed multiple GMTs; a relatively sensitive GMT to span the lower part of the dynamic range and a less sensitive GMT to span the upper decades.
Using the time-to-count algorithm, the rate, r, is calculated as follows. First, calculate the total time, tActive, the GMT is active:
      t    Active    =            N      t              f              o        ⁢                                  ⁢        s        ⁢                                  ⁢        c            where Nt is number of clock ticks and fosc is oscillator frequency. Next, calculate the mean time to count, μtime to count:
            μ      timetocount        =                  t        Active                    N        p                  r    =          1              μ        timetocount            where Np is number of clock ticks to pulse measurement and rate, r, may be calculated as:
      ∴    r    =            f              o        ⁢                                  ⁢        s        ⁢                                  ⁢        c              *          (                        N          p                          N          t                    )      
U.S. Pat. No. 5,206,513, the contents of which is incorporated herein by reference in its entirety, addresses the limitations of the conventional GMT counters summarized above and describes a portable radiation measurement device using a single GMT that has an operating range from background radiation levels on the order of 20 μR/h to high radiation levels on the order of 1000 R/h. U.S. Pat. No. 5,206,513 extends the measurement range of a single Geiger Mueller tube (GMT) using a system that may be operated in one of two modes; either a conventional mode, used to measure low radiation levels, e.g. background radiation, or an extended range Geiger (ERG) mode, used to measure high radiation levels. Both modes utilize the same GMT and basic operating circuitry, including a power supply for generating a GMT anode voltage, a GMT anode voltage control circuit, a GMT trigger circuit, a clock circuit, and a GMT pulse counter circuit. In the conventional mode, the radiation rate is determined as a function of the number of GMT pulses counted over a prescribed time period, i.e., conventional pulse counting. In the ERG mode, an additional counter is employed to count trial intervals of a prescribed duration. The radiation rate is determined in the ERG mode statistically as a function of the ratio of the GMT pulse count and the trial interval count, with a prescribed number of trial interval counts being rejected after each GMT pulse in order to assure known initial conditions.
Using the conventional pulse count algorithm, the number of pulses Np is measured over a period of time and rate, r, is determined using:
  r  =            f              o        ⁢                                  ⁢        s        ⁢                                  ⁢        c              *          (                        N          p                          N                      c            ⁢                                                  ⁢            l            ⁢                                                  ⁢            k                              )      Using the ERG algorithm, first calculate the probability of a non-event as follows:
      P    ⁡          (              >        0            )        =                              N          p                          N          t                    ⇒              P        ⁡                  (          0          )                      =          1      -                        N          p                          N          t                    Apply Poisson arrival statistics:
            P      ⁡              (                  x          ,          λ                )              =                            λ          x                ⁢                  ⅇ                      -            λ                                      x        !                  λ    =          r              f                  o          ⁢                                          ⁢          s          ⁢                                          ⁢          c                                P      ⁡              (        0        )              =                                                                      (                                  r                                      f                                          o                      ⁢                                                                                          ⁢                      s                      ⁢                                                                                          ⁢                      c                                                                      )                            0                        ⁢                          ⅇ                              -                                  r                                      f                                          o                      ⁢                                                                                          ⁢                      s                      ⁢                                                                                          ⁢                      c                                                                                                                0            !                          ⇒                  P          ⁡                      (            0            )                              =              ⅇ                  -                      r                          f                              o                ⁢                                                                  ⁢                s                ⁢                                                                  ⁢                c                                                        And calculate rate as follows:
      ∴    r    =            f              o        ⁢                                  ⁢        s        ⁢                                  ⁢        c              *          ln      ⁡              (                              N            t                                              N              t                        -                          N              p                        +            ɛ                          )            For each rate measurement algorithm described herein, the field strength, Rgamma, may be determined as follows:
      R    gamma    =      r          Tube      ⁢                          ⁢      Sensitivity      
The prior art systems described above face a common challenge and trade-off in implementation with respect to power consumption. More particularly, the upper limit of the radiation measurement algorithms (or modes) is a function of the GMT sensitivity to radiation and the frequency of the clock oscillator. Since power consumption increases linearly with clock frequency, the multiple GMT systems with single measurement mode, varying sensitivities are used to obtain a large dynamic range and maintain a low clock speed. This multiple GMT system reduces the clock speed required to measure high radiation fields but increases the size, weight and complexity of the device since multiple GMTs are required. The single GMT based, multiple measurement mode systems such as that described in U.S. Pat. No. 5,206,513 have lower size and weight but require more power to drive a higher clock rate to meet the full dynamic range requirement.
Accordingly, there remains a need in the art for a GMT-based radiation detection system which uses a single GMT to reduce size and weight while also reducing power.